I've dug out my relic of a CBFM to start to track my cycles in advance of potentially trying for DC3. I used it last month and had high reading from CD18 and peak at CD21. This month I'm on CD23 and nothing, no high or peak! Early on in my cycle I missed a day and I also missed a day around 17/18/19 (can't remember exactly which day it was).
Could missing days have messed up my readings?